1. Field of the Invention
The present invention relates to a memory card which includes memory devices such as a RAM and a ROM for storing data or software.
2. Description of the Related Art
Illegal copying is a serious problem to software suppliers. However, in the present situation in which floppy disks are usually employed to supply software, software suppliers find it difficult to take active measures to prevent illegal copying when the convenience of many authorized users is taken into account. This is because users need to copy software from floppy to RAM disks or hard disks and because back-up copies are needed because of the low access reliability of floppy disks.
As is known, memory cards which include semiconductor memory ICs such as RAMs and ROMs have access speed and access reliability, both of which are extremely high compared with those of hard disks. For this reason, even if active measures to prevent illegal copying are incorporated into memory cards in which software is stored, authorized users will not suffer inconvenience. Accordingly, if the feature of such a memory card, that is, the incorporation of measures to prevent easy illegal copying, is emphasized in contrast to the problem, of floppy disks it will be possible to provide a stepping stone to the enlargement of the range of applications of memory cards.
FIG. 1 is a block diagram of a protection system for use with software which is supplied by means of a conventional type of floppy disk. As illustrated, a floppy disk 2 in which coded or scrambled software is stored is connected to a personal computer 1 by a signal line 3. A software protecting device 4 available under the tradename of "PROTECT BOX" or "LOCK STAR" is connected to the personal computer 1 by a signal line 5. The signal line 5 is an input/output signal line according to the RS232C or CENTRONICS interface standard, which enables the protecting device 4 to be easily connected to the personal computer 1. The operation of the software is possible only when the protecting device 4 is connected to the personal computer 1. If the personal computer 1 detects the disconnection of the protecting device 4 during the operation of the software, the personal computer 1 interrupts the execution of the software.
FIG. 2 is a block diagram showing the construction of an internal basic circuit of a conventional IC memory card (ROM card), described in "IC Memory Card Guideline" (issued in September, 1986) compiled by the personal computer work committee of Nippon Denshi Kogyo Shinko Kyokai (Japan Electronic Industry Promoting Association, an incorporated body). In FIG. 2, a memory chip section 8 is normally composed of a plurality of ROM chips (refer to FIG. 3). An interface connector 6 is a connector for providing a connection with an external device. A low-order address bus 11 and a data bus 12 which extend from the interface connector 6 are connected directly to each memory chip in the memory chip section 8. A chip select circuit 7 serves to select a specified memory chip in the memory chip section 8, and a control bus 9 and a high-order address line 10 which extend from the interface connector 6 are connected to the chip select circuit 7. An external power source line 13 for supplying electrical power from a power source (not shown) disposed outside the memory card is connected to both the memory chip section 8 and the chip select circuit 7.
FIG. 3 is a block diagram showing in greater detail the memory chip section 8 and the chip select circuit 7 both of which are particularly associated with the present invention in the block diagram of FIG. 2. In FIG. 3, like reference numerals are used to denote like or corresponding portions shown in FIG. 2. As illustrated, the chip select circuit 7 is constituted by, for example, a general-purpose gate IC "74HC138", while each ROM chip 81 and 82 is a ROM chip, such as "27C256", each having a storage capacity of 32K bytes. A card select line [CE] 91 which serves to select a particular memory card and place it in an enable state extends from the interface connector 6 (refer to FIG. 2)to the chip select circuit 7. Chip select lines [S1, S2] 93 and 94 extend from the chip select circuit 7 to the ROM chips 81 and 82, respectively. An output enable control line [OE] 92 is connected to both the ROM chips 81 and 82. In the arrangement shown in FIG. 3, the high-order address signal of the high-order address line 10 is composed of a 1-bit signal indicative of a high-order address A15 (the number of bits is determined by the number of memory chips constituting the memory chip section), while the low-order address signal of the low-order address bus 11 is composed of a 15-bit signal indicative of low-order addresses A0 to A14 (since the memory capacity of each of the memory chips is 32 k bytes). The data signal provided at a bidirectional data bus 12 is composed of a 8-bit signal corresponding to data lines D.sub.0 -D.sub.7. In FIG. 3, for simplicity of explanation, the memory chip section 8 is shown constituted by the two ROM chips 81 and 82 but, in general, the memory chip section 8 is constituted by an even larger number of memory chips. Even if the number of memory chips is increased, the basic operation of the memory card itself does not change, and the number of high-order address lines 10 and the number of chip select lines 93, 94 extending from the chip select circuit 7 merely increase.
However, since a conventional type of memory card is constructed in the manner described above, there has been a problem in that it is possible to easily read internally stored software into an external device or to easily copy it onto a recording medium such as another floppy disk. Furthermore, in terms of the conventional circuit construction, it has been difficult to incorporate measures to prevent copying.